The present invention relates to a method of manufacturing a diamond composite and a diamond composite produced thereby. This patent application is related to PCT patent application Nos. PCT/EP98/04414 and PCT/EP98/05579, and to Russian Patent Application Nos. 98118300 filed Sep. 28, 1998 and 99100821 filed Jan. 26, 1999, which are hereby incorporated by reference.
There is a general need of extremely hard materials for many fields of application. These extremely hard materials are also called xe2x80x9csuperhardxe2x80x9d when they exhibit a hardness of  greater than 40 GPa. These materials are used in a variety of applications such as tools for cutting, turning, milling, drilling, sawing, grinding operations, and the like. The hard materials may also be used for their wear, abrasion and erosion resistance when working as bearings, seals, nozzles or in similar cases. The materials may be working on, or being in contact with, many materials such as cast iron, steel, non-iron metals, wood, paper, polymers, concrete, stone, marble, soil, cemented carbide and grinding wheels of aluminum oxide, silicon carbide, diamond, cubic boron nitride, and the like. As being the hardest material known, mono- or polycrystalline diamond is suitable for these purposes. Other common materials used for their hardness are for instance cubic boron nitride (CBN), boron carbide and other ceramics and cemented carbides, however only diamond or CBN containing materials can reach the superhard group of materials.
It is well known that carbon in the diamond structural form is thermodynamically unstable at ambient temperatures and pressures. Nevertheless the decomposition of diamond to graphite (graphitization) is hindered by kinetic reasons and diamonds found in nature have existed for millions of years. However, by increasing the temperature, graphitization of diamond crystals will occur with a process starting from the surface, where the energy to overcome the kinetic hindrance is highest and where defects or catalytic effects from other surface impurities or the atmosphere will influence this process.
By heating in air it is well known that the decomposition and oxidation of diamonds will take place at temperatures as low as 600-700xc2x0 C. Carbon solving metals like cobalt may catalyze a reaction already at about 500xc2x0 C. The graphitization process is delayed to higher temperatures in vacuum or inert atmosphere and diamonds are most stable in hydrogen gas atmosphere, where the environment is strongly reducingxe2x80x94High quality diamond is stable for long times to about 2000xc2x0 C.
Different composite bodies with bonded diamond particles are known. The diamond particles may be bonded by a matrix comprising metal and/or ceramic phases and produced by sintering diamond particles in a matrix of such materials, or bonded by the infiltration of silicon or silicon alloys into the diamond body, for instance.
By heating a body of diamond powder in a furnace to high temperatures during extended times, a small amount of uncontrolled and undesirable graphitization might occur depending also on the pressure. In previously reported processes to form densely sintered diamond composite bodies this has been an unwanted effect and different ways of avoiding this have been used. The most practiced technique is to use high pressures during the sintering step and stay in the diamond stable area of the phase diagram at 1300-1600xc2x0 C., in high-pressure chambers with pressures of 30.000-60.000 atm (HP/HT). See for instance FIG. 4, in U.S. Pat. No. 4,151,686; for a diamond-graphite phase diagram.
The required extremely high pressures are only achieved by specially made presses and dies. The consequences are high production costs, limited production capacity and limited shapes and sizes of the diamond composite bodies.
There are also methods for production of diamond bodies using lower pressures than needed for the diamond stable area, from about a minimum of 500 psi (about 34 bars) and above, e.g. the method according to U.S. Pat. No. 4,124,401.
In the case where the pressure has been in the graphite stable region, for instance using a furnace with protective inert atmosphere, graphitization has been minimized by using short times at high temperature or reducing the sintering temperature for solidification of the body. An example of the latter is to use metal alloys of silicon that have a significantly lower melting temperature than that of pure silicon.
Several patents reveal techniques to produce materials containing diamond, silicon carbide and silicon without using high pressures. There are a number of variations of the process, mainly concerning the use of different carbonaceous materials (hereafter referring to all kinds of non-diamond carbon materials like carbon black, carbon fibres, coke, graphite, pyrolytic carbon etc). In principal the following steps are followed.
A. Non-coated diamond particles or normally, carbon-coated diamond particles and carbonaceous materials are used as precursor materials. According to the examples, U.S. Pat. No. 4,220,455 starts with adding a thin layer (500-1000 Angstrom) of carbon on the diamonds by a pyrolytic reaction. The pyrolysis is done in vacuum for a few minutes by feeding natural gas or methane, into a furnace with diamond particles at 1200xc2x0 C. Sometimes diamonds without a pyrolytic carbon layer are used, as in U.S. Pat. No. 4,381,271, EPO 0 043 541, EPO 0 056 596 and JP 6-199571A. Both carbon-coated and non-coated diamonds are mixed with carbonaceous materials as a main source of carbon e.g. carbon black, short carbon fibres or cloth and a binder etc. before the forming of green bodies.
B. Forming of green bodies of the diamond particle/carbon material mixture is done in a mould. The green bodies contain additionally solvents and temporary or permanent binders to facilitate the forming and to increase the strength of the green body.
C. Work-pieces are made by heat treating the green bodies. Some binders are vaporised without leaving any residues e.g. paraffin, other binders are hardened leaving a carbonaceous residue in the work-piece, e.g. phenol-formaldehyde and epoxy resins.
D. Infiltration of the porous work-piece with molten silicon is done to form silicon carbide in a reaction between the carbon and the silicon. The heat treatment is done in such a manner as to minimise the graphitization of diamond, which is considered harmful. In the examples of U.S. Pat. No. 4,220,455 silicon is infiltrated in vacuum when the body is in a mould, at a temperature between 1400-1550xc2x0 C. for 15 minutes, during which time the reaction between silicon and carbon is completed. U.S. Pat. No. 4,242,106 uses a vacuum of 0,01-2,0 torr during the infiltration. The required time, depending largely on the size of the body, is determined empirically and takes about 15-20 minutes at a temperature above 1400xc2x0 C., or 10 minutes at 1500xc2x0 C. U.S. Pat. No. 4,381,271 uses carbon fibres to promote the infiltration of fluid silicon by a capillary action. In most of the patents infiltration is made in a mould. In some earlier patents the infiltration is made outside the mould, like in EPO patent 0 043 541.
Not only silicon has been used for the infiltration and bonding of diamond particles. Several patents describes using silicon alloys instead of pure silicon. U.S. Pat. No. 4,124,401 describes a hot-press method using an eutectiferous silicon alloy for infiltration. U.S. Pat. No. 5,266,236 uses aboron-silicon alloy in a HP/HT method. U.S. Pat. No. 4,664,705 discloses a method that infiltrates a silicon alloy through a PCD body, where the binder has earlier preferably been leached out.
The processes where carbon-coated or non-coated diamonds are mixed with carbonaceous materials might have disadvantages, e.g. difficulties in preparing homogeneous mixtures of these materials, difficulties of silicon infiltration due to very small pore sizes and necessity of special equipment for preparing homogenous mixtures.
In the patent RU 2064399 the addition of carbon by pyrolysis is done only after the forming and production of the work-piece. A preformed work-piece of diamond particles or a mixture of diamond particles and carbide grains as filler, is produced with a temporary binder. The binder is evaporated and the work-piece is placed in a reactor, where pyrolytic carbon is deposited on all grains of the body by a pyrolytic reaction from a gas phase, e.g. methane at 950xc2x0 C. for 10 h. After this follows silicon infiltration. The drawbacks of this process are the use of a great amount of hydrocarbon gas and that the processing time is rather long. If carbide grains are used as fillers, the same problems of homogenisation as mentioned above appear.
There are some methods for improving the diamond composite materials produced by the earlier described techniques. One of them is to arrange the diamond particles as graded structures of concentration and size in the material, thereby affecting some properties and also the field of application. A method of making a size graded material by sintering at high pressure and high temperature is disclosed in the patent EPO 0 196 777. The grain size and/or packing density are varied in layers between the front face and rear face to get different wear resistance in these parts. The drawback of this method is that since it uses high pressure-high temperature, the production of the material is more expensive and requires special equipment and there are size limitations.
There are also a number of patents using different amount of diamond in different parts of the composite body. The following patents U.S. Pat. No. 4,242,106; U.S. Pat. No. 4,247,304; U.S. Pat. No. 4,453,951; EPO 0 043 541; EPO 0 056 596 describe the production of layered structures of a final material with a diamond composite layer in contact with a supporting silicon carbide or silicon carbide-silicon substrate, for instance. U.S. Pat. No. 4,698,070 describes the production of a composite with a diamond containing portion and a core portion united by a matrix of silicon carbide and silicon. Additional particle layers with other diamond concentration may also be provided and placed e.g. in comers, on the top, in the core.
Generally the drawback of layered materials with different diamond size or concentration is that there may be differences in physical/mechanical properties in the diamond containing and supporting layers, e.g. thermal expansion coefficient and E-modulus, might cause unwanted stress situations at the interface and thereby weaken the composite under stress. Diamonds have a relatively low tensile strength and low toughness, and a distinct difference in diamond content in different parts joined by an interlayer may also affect the fracture resistance of composites. None of the methods described earlier result in bodies with prior specified distribution of diamond particles of different size throughout the material volume, with uniformly changing properties.
The composites of U.S. Pat. No. 4,220,455 consist of a mixture of diamond particles of different size in the whole body, i.e. the composite does not have layered structures. The particular sizes used are chosen depending on the desired packing of particles and resulting body. For most abrasive applications particles no greater than about 60 xcexcm are preferred. Preferably to maximise the packing of the particles they should contain a range of sizes, i.e. small, medium and large.
None of the methods described above use graphitization intentionally. Instead the graphitization is considered as harmful and unwanted.
In RU patent 2036779 a preform is moulded of diamond powder eventually together with water or ethyl alcohol, placed in a furnace and impregnated with liquid silicon at 1420-1700xc2x0 C. in argon or vacuum. In the process the surface of the diamond grains is minimally graphitized, so the greater part of the diamond is still unchanged. This minor amount of graphite, reacts in contact with infiltrated silicon creating a thin surface layer of silicon carbide that keeps back any further formation of diamond to graphite during the used process. The drawback of this process is poor control and there is no method for governing the amount of produced SiC, residual silicon or porosity left in the composite.
Thus in these previous patents there is no teaching about a well-controlled step of adding carbonaceous materials to a diamond body and intentional graphitization step for production of materials with desired amount of diamond, silicon carbide and silicon, with low porosity and no graphite.
In contrast to these previous approaches, one important step in the process to prepare a diamond composite according to the present invention is to use a desirable and controlled graphitization that deliberately transforms a layer of intended thickness at the surface of the diamond particles to graphite. Graphitization is a complex process depending not only on the important time-temperature curve of the process, but also on the diamond particle size, type and quality of diamond, presence of catalytic impurities, the atmosphere, presence of oxygen, pressure etc. Smaller particles have a larger relative surface area than coarser particles and surface defects and type of diamond are all important parameters. Presence of carbon-soluble metals like cobalt, nickel or iron and the presence of oxygen or oxidizing atmosphere (e.g. carbon monoxide) will have a great influence. Therefore, for a certain starting material, furnace and given process parameters it is important empirically to carefully determine the degree of graphitization. This knowledge will provide the background for an appropriate time-temperature curve for governing the graphitization in a controlled and safe way during production.
In the present inventive process, by changing the relative amount of graphite in the diamond body before the infiltration of silicon or silicon alloy melt, it is possible to prepare a desired phase composition, microstructure and, subsequently, control the material properties. The graphite layer on the diamond particles shall have uniform coverage. The minimum amount of graphite in such layers should allow formation of strong chemical bonding by SiC formation between diamond interfaces and the matrix. The amount of formed SiC shall also be enough to form a tight protective layer. For micron sized or larger diamond particles the graphitization should be at least more than approximately 3 wt-% and preferably lie between approximately 6-30 wt-%, as discussed in detail below.
In most diamond composite bodies produced in the prior art it has been attempted to use very high diamond concentrations to form a direct chemical bonding between the diamond particles, i.e. giving a diamond skeleton structure. This has been supposed to increase the mechanical strength and rigidity of the composites. Surprisingly we have found that such a direct bonding is not needed to achieve good mechanical properties. A direct bonding of the diamonds is not an important or needed factor in our diamond composites, although at the highest diamond concentrations some diamond to diamond contact might occur.
In the process according to the present invention in case of using pure silicon as the infiltrate melt into a diamond body, the products besides diamond will be silicon carbide and residual silicon filling the porosity and resulting in a fully dense body. Materials properties like hardness, toughness and rigidity will be influenced by the amount, distribution and particle size of the different phases.
However, by using a silicon alloy a more complex material will be formed with wider possibilities to prepare materials with desired overall properties for different applications. Besides the phases mentioned above the alloying element could form either carbides with the non-diamond graphite present at the initial stage of the process or form a metal silicide. Residual silicon alloys of varying composition (or even silicon) will be present or small amounts of metal carbosilicides might form.
Boron carbide (B4C), which is harder than silicon carbide will form resulting in a harder final body, when using boron as an alloying element in silicon. Other strong carbide formers like Ti, Zr, Nb and Ta are predicted from Gibbs energy calculations to form metal carbide rather than metal silicide. The presence of these carbide particles in the microstructure could increase the toughness and not deteriorate high temperature properties. However, kinetic factors might cause some silicide formation. The presence of metal silicides will increase the toughness at low and medium temperatures, but some silicides like those from the iron group will not be beneficial for high temperature use above 1000xc2x0 C. Other silicides like molybdenum disilicide are known to have good high temperature properties especially in air where initial oxidation forms a silica layer protecting from further oxidation.
The process according to the present invention is a low-pressure process considerably below the pressures required for the diamond stable region and will allow low-cost mass production also of large and/or complex bodies. A novel feature of our production process is that it does not need special presses and dies. For example we do not need to use expensive hot isostatic pressing (HIP) equipment for gas pressures up to 2 kbar. In this case, both the HIP equipment and running costs of the process are very high and the process requires a gas-tight metal, glass or other encapsulation for transferring the pressure to the bodies to be sintered. Stringent safety precautions are in force when using such high gas pressures, and during operation and maintenance of such equipment.
Hot pressing (HP) equipment is available at lower costs where pressures typically from 30 to 1500 bars, are applied to the diamond body by graphite punches during sintering. The production capacity is limited and the sintered bodies are most likely in the form of discs or plates. Complex shaped bodies for engineering purposes cannot be prepared easily with prior art methods. The present invention avoids these limitations.
From a production cost point of view the pressure used should be below approximately 50 bar, preferably below approximately 30 bar. At this pressure very much simpler production equipment can be used and complex shapes can be made.
The lowest production cost and large-scale manufacture is achieved with furnaces using ambient pressure of inert gas or a slight overpressure less than 2 bars. Vacuum can also be used. High production capacity lowers costs drastically and the sizes of the composite components can be increased.
The use of nitrogen as a low-cost inert gas is possible in the latter case as a low pressure gas. However, increasing nitrogen pressure above 2 bars at the melting temperature of silicon or silicon alloys might cause a dramatic reaction between silicon and nitrogen to form silicon nitride. This reaction is strongly exothermic and once started in might be uncontrolled increasing local temperatures destroying the diamonds and the composite.
The principle object of the present invention is the process for making diamond composites having excellent properties, and the superhard material produced thereby. The method should be easily performed, fast and cost effective and offer possibilities to control several properties and cost of the final materials.
An object of the invention is obtained by a low pressure method for manufacturing a diamond composite, where the diamond particles are bonded by a matrix comprising silicon carbide and silicon, or alternatively combinations of the following material phases; silicon carbide; other carbides such as metal carbide or boron carbide; silicon; metal silicides, metal carbosilicides and/or silicon alloys; comprising the steps of forming a work piece, heating the work piece and controlling the heating temperature and heating time so that a certain desired amount of graphite is created by graphitization of diamond particles, thereby creating an intermediate body, and infiltrating silicon or alternatively a silicon alloy into the intermediate body.
In a preferred embodiment the amount of graphite created by graphitization is approximately 1-50 wt-%, preferably 6-30 wt-% of the amount of diamond and the heating temperature during graphitization is lower than 1700xc2x0 C. The heating temperature and heating time needed for the graphitization is empirically determined for the heating equipment used. The work piece is formed with a porosity of approximately 25-60 vol-%.
In a another embodiment of the present invention a certain amount of carbon is deposited in the work piece by exposing it to a gaseous hydrocarbon or gaseous hydrocarbons at a temperature exceeding the decomposition temperature for hydrocarbon or hydrocarbons, and at least some graphitization of the diamond crystals is done before exposing the work piece to a gaseous hydrocarbon or gaseous hydrocarbons at a temperature exceeding the decomposition temperature for hydrocarbon or hydrocarbons. The intermediate body can be machined into the desired shape and size of the final body before the step of infiltration of liquid silicon or silicon alloy.
In a further embodiment the intermediate body is heated together with silicon or silicon alloy that is vaporised, and the body is then machined into the desired shape and size of the final body before the step of infiltration of liquid silicon or silicon alloy.
Infiltrating a silicon alloy into the intermediate body is characterized in that the melt is silicon alloy comprising at least one metal from the group consisting of: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Ag, Al, or the element B or Ge. When referring to metals Germanium (Ge) is considered as a metal. The heat treatment and infiltration of the diamond containing work-piece are performed at a pressure of less than approximately 50 bars of an inert gas, preferably below 30 bars, and most cost efficient below 2 bars inert gas pressure or in vacuum. The infiltration is carried out at temperatures higher than the melting temperature of the melt, i.e. at temperatures higher than 1450xc2x0 C. for most of the silicon alloys or at temperatures higher than 1100xc2x0 C. when using alloys containing Al, Cu, Ag, Fe, Ni, Co, Mn, or Ge. The temperature of the infiltration should be kept less than 1700xc2x0 C., preferably less than 1600xc2x0 C.
It is possible to make silicon alloys with the aforementioned alloy metals or boron or germanium. Their liquidus temperatures are low, which is important. The graphitization process is better controlled and these silicon alloys have moderate vapor pressure at temperatures in the interval 1200-1700xc2x0 C. Finally, elements from the selected alloying elements form additional phases in the material which gives the diamond composite valuable properties. These phases can be metal carbides, metal suicides, metal alloys with silicon or ternary metal carbosilicides, or the corresponding phases of boron.
Good results are obtained when using silicon alloys, in which content of alloying element in the silicon allow is as follows:
from Ti, Zr, or Hf is less than 50 wt-%, preferably less than 20 wt-%.
from V, Nb, or Ta is less than 20 wt-%, preferably less than 10 wt-%.
from Cr and Re is less than 45 wt-%, preferably less than 20 wt-%.
from Mo and W is less than 10 wt-%, preferably less than 5 wt-%.
from Mn, Fe, Co, or Ni is less than 60 wt-%, preferably less than 20 wt-%.
from Cu and Ag is less than 30 wt-%, preferably less than 15 wt-%.
from Al and Ge is less than 50 wt-%, preferably less than 20 wt-%.
from B is less than 20 wt-%, preferably less than 8 wt-%.
Infiltration of liquid silicon alloys into the intermediate body is performed by the most suitable methods, for example, by melting of the corresponding alloy directly on the surface of intermediate body, or, for example, by dipping of intermediate body in the corresponding melt or, for example, by pouring of the corresponding melt on surface of intermediate body. When using alloys it ensures simple infiltration process connected with the lower melting temperature of alloys compared with individual substances, better wetting of the intermediate body surface, a lower viscosity and more intensive penetration into pores of the intermediate body. As a result of infiltration a practically non-porous material comprising diamond, silicon carbide and additional phases, content of which is determined by the type of the used metals in the alloy, is produced. Such additional phases can be metal silicides (for example NiSi2) and/or metal carbides (for example TiC and NbC) and /or alloys of metals (for example Ag) with silicon.
Content of metals (besides silicon) in the final dense diamond composite is less than approximately 30 wt-%, preferably less than 20 wt-%. For some metals the content is naturally limited by the composition of the used silicon alloy and the maximal porosity of the initial work-piece. Thus, for metals from the group of V, Nb, or Ta their content in material is less than 10 wt-%, preferable less than 5 wt-%. For metals from the group of Mo and W their content in material is less than 5 wt-%. Finally, for the metals Fe, Co, and Ni the material content should preferably be less than 10 wt-%.
The work piece can be formed with a uniform or non-uniform distribution of diamond particles with various sizes and qualities. For instance, the diamond particles in the work piece can be distributed in successively decreasing sizes from the surface of the work piece towards the centre thereof. The work piece can in a variant be formed from a homogeneous mixture of diamond crystals of various sizes eventually with the addition of a binder.
In yet another embodiment two or more work pieces are made separately and thereafter being brought together before the heat treatment and the infiltration steps.
The forming of the work piece may be made in a mould, the heat treatment and the infiltration of silicon or alternatively silicon alloy being made after the work piece has been taken out of the mould.
The forming of the work piece may be made in a mould, the heat treatment and the infiltration of silicon or silicon alloy being made having the work piece in a mould.
The invention also relates to a body in which diamond particles are bonded to a matrix of silicon carbide, said body comprising at least approximately 20 vol-% of diamond particles, at least 5 vol-% of silicon carbide, preferably more than 15 vol-% of silicon carbide, and silicon or other metal-silicon-carbon or boron-silicon-carbon phases, the Young""s modulus exceeding 450 GPa.
In another embodiment, said body comprising at least approximately 29 vol-% of diamond particles, at least approximately 14 vol-% of silicon carbide, and silicon or other metal-silicon-carbon or boron-silicon-carbon phases, the Young""s modulus exceeding 540 GPa.
In a preferred embodiment, said body comprises at least approximately 46 vol-% of diamond particles having sizes of about 30 xcexcm at the most, the Young""s modulus exceeding 560 GPa.
In another preferred embodiment, said body comprises at least approximately 54 vol-% of diamond particles, at least 60 vol-% of the diamond particles having sizes of at least 50 xcexcm, the Young""s modulus exceeding 650 GPa.
In all these embodiments the body maintains its shape and its Young""s modulus up to a temperature of at least 1500xc2x0 C. in vacuum.
In a further embodiment, diamond particles of sizes of about 10 xcexcm or less are embedded and included in the matrix, the Vickers microhardness of the matrix measured in the area between the diamond particles being greater than 30 GPa for a load of 20 N, the Knoop macrohardness of the matrix being greater than 36 GPa for a load of 20 N.
In another embodiment the diamond particles have one size fraction of particles being larger than about 50 xcexcm and one sizes fraction of particles having a size of 50 xcexcm at the most, the mass ratio falling in the range of about 0.25 to 2.5 and the mean particle size being larger than 10 xcexcm, preferably larger than 20 xcexcm.
In yet another embodiment the diamonds have one size fraction of particles being large diamond particles and one size fraction being small diamond particles, the mass ratio falling in the range of about 0.25 to 2.5 and the mean particle size being larger than 10 xcexcm, preferably larger than 20 xcexcm.
In a further embodiment the diamond particles have one size fraction being large diamond particles and one size fraction being small diamond particles, the abrasion rate being less than approximately 26 xcexcm3/m, preferably less than 10 xcexcm3/m (example 10).
In a further embodiment the diamond particles have one size fraction being large diamond particles and one size fraction being small diamond particles, the erosion rate being less than approximately 0.34 mg/g, preferably less than 0.25 mg/g (example 10).
In a further embodiment the diamond particles have sizes less than about 20 xcexcm, the abrasion rate being less than 26 xcexcm3/m, preferably less than 10 xcexcm3/m (example 10).
In a further embodiment the diamond particles have sizes less than about 20 xcexcm, the erosion rate being less than 0.34 mg/g, preferably less than 0.25 mg/g (example 10).
In a variant of the embodiments the body is hollow.
In a further embodiment a surface of the body is coated with diamond film.
In yet another embodiment, the body comprises large diamond particles of a size larger than 20 xcexcm, the matrix comprising 0-50 vol-% of small diamond particles having sizes less than 20 gm, 20-99 vol-% of silicon carbide, and 1-30 vol-% silicon or other metal-silicon-carbon or boron-silicon-carbon phases, the matrix hardness being 20-63 GPa.
In a first variant, the matrix hardness is 20-30 GPa. In a second variant, the matrix hardness is 50-63 GPa. In a third variant, the matrix hardness is 30-50 GPa.